This study seeks to improve our understanding of self-regulation and its role in emerging mental health problems by utilizing neurocognitive and physiological measures of self-regulation and examining their co- development during a period of marked plasticity (both neural and behavioral) and rapid CMS development. Effortful control (EC) and respiratory sinus arrythmia (RSA) are indicators of cognitive and emotion regulation, respectively. Despite solid evidence that these components of self-regulation are interrelated, there is almost no research examining the co-development of EC and RSA in young children. Specifically, there is little research examining how EC and RSA develop, predict, and potentially shape one another over time. Examining how individual differences in the development of EC predict changes in RSA, and vice versa, might clarify how particular deficits of attention, behavioral and/or emotional regulation develop. Low EC and RSA have been linked to mental health problems including anxiety, depression, and externalizing problems. Examining how neurocognitive and physiological indicators of self-regulation predict concurrent and later mental health problems in young children may identify key mechanisms in the development of psychopathology, and point to potential targets of intervention. The proposed research will assess 174 families with 3 year old children across three time points. Two thirds of the sample will be families of low-income or poverty backgrounds. Strains associated with low- income and poverty environments will increase the incidence of emotional and behavioral problems in children and observation of atypical self-regulatory development.